Thanks you all for your replies!
From your responses, it looks like I should do the following things in this order:
1. Check for leaks at each caliper (and anywhere in between).
I suspect that this will not be the problem, as the fluid level appears ok in the reservoir, and I detected no leakage a few months back when I changed the pads. But I will do a thorough check and replace/repair any visible leaking component.
2. Check for vacuum leak or bad vacuum pump.
I'm a little foggy on this and what this involves. If I use Nello's test (pump pedal, and crank engine) and the test is negative, I'm assuming that it is NOT the vacuum pump, but could it still be the brake boost (diaphragm) that might be bad? There seems to be several different brake boost/diaphragm kits available.
3. Replace master cylinder.
If Nello's test above is negative, should I jump to this step?
I probably am looking at this with wishful-thinking as the third step seems to be the least expensive, judging from the prices I can find here in the Fastlane. I know that I shouldn't just replace the master cylinder, if I really have a vacuum problem.
Thanks again for all your help.